The fate of a T lymphocyte is determined by the ligand specificity and affinity of its receptor. This is clearly the case during development in the thymus, when cells are carefully selected to avoid autoreactivity. However, in the interest of maintaining an optimally diverse repertoire, not all cells with anti-self reactivity are deleted, and those with low affinity receptors, or cells that have downregulated their avidity by reduced levels of expression of receptor or co-receptor are permitted entry into the periphery. These represent a pool of potentially autoreactive cells. The overall goal of this proposal is to learn more about such cells and how T cell avidity and TCR affinity determines their participation in autoimmune diabetes as well as in a conventional immune response. To achieve these goals we will utilize a transgenic model in which the influenza hemagglutinin(HA) is expressed under the control of the rat insulin promoter, resulting in high level expression of HA on pancreatic beta cells, and low level expression in the thymus. These Ins- HA animals are tolerant of the transgene, however they retain CD8+ T cells that respond to HA with low avidity. Cloned T cell lines will be developed and analyzed to determine the mechanism utilized to downregulate avidity in a conventional T cell repertoire. The affinities of the TCRs expressed by these clones will be compared. The potential of clones to cause diabetes will be correlated with their phenotype, cytokine expression, TCR affinity, and the mechanism used to achieve low avidity. TCR transgenics will be made that express different affinity receptors in order to determine how affinity alters the fate of T cells during development in the Ins-HA animals, and to compare conditions under which these cells are activated by antigen to cause diabetes. Finally, TCR transgenics that express different affinity receptors will be compared with respect to their response potential of the cells during an immune response to influenza. We will test the hypothesis that the degree of clonal expansion and memory cell generation is correlated with the affinity of the cell for antigen, thus resulting in a higher affinity anamnestic response to antigen.